Chemical vapor deposition (CVD) is a deposition process that is used to form thin films on a substrate, such as a wafer. In a CVD process, a substrate is exposed to one or more precursors in a reaction chamber. The substrate is typically heated to a temperature higher than the decomposition temperature of the precursor so that when the precursor contacts the substrate it reacts with or decomposes onto the surface of the substrate to produce the desired thin film. However, the crystallographic structure of some substrates may affect the growth of the film, producing non-planar, three-dimensional films due to preferential growth rates along different crystallographic axes. For example, in hexagonal wurtzite materials, such as zinc oxide (ZnO), gallium nitride (GaN), and related materials, growth along the two dominant axes, the c-axis and a-axis, varies primarily due to the anisotropy in growth rates along these two axes. Some techniques have been developed to enable single crystalline and contiguous growth on the c-axis. See, e.g., PCT publication WO 2008/073469 (PCT application serial number PCT/US2007/025432, filed Dec. 11, 2007) entitled “Zinc Oxide Multi-Junction Photovoltaic Cells and Optoelectronic Devices,” commonly owned by the assignee and incorporated by reference herein. Nevertheless, planar deposition of ZnO and ZnO based alloys on substrates with hexagonal crystallographic structures remains a challenge.